Conventionally, such a dielectric filter is used as a filter for the microwave band and the millimeter-wave band, and, in particular, a waveguide type filter in which a structure is provided in its waveguide or a dielectric resonator filter is often used. FIG. 23 shows perspective views of a waveguide type filter 101 as one example of the waveguide type filter.
As shown in FIG. 23B, the waveguide type filter 101 obtains a filter characteristic by placing a metal plate 104 where a plurality of windows 104a are formed between separate waveguides 102 and 103 that have mutually paired shapes and are able to internally form one waveguide by being joined together, joining the waveguides together, and positioning the windows 104a in the waveguide as shown in FIG. 23A. Such a waveguide type filter 101 has the features that it is a transmission line of low loss particularly in the millimeter-wave band (30 to 300 GHz) and the Q value (Quality Factor) of the resonator is also large. On the contrary, there is a problem that the size reduction of the waveguide type filter 101 is difficult because a waveguide-to-microstrip conversion is needed when power is fed by a microstrip line.
Moreover, in the dielectric resonator filter, by placing a filter element formed of a dielectric in a housing made of a metal and feeding powder directly by means of a waveguide or feeding power by means of a microstrip line or the like, electromagnetic waves at the desired frequency are made to resonate in the metal housing, and electromagnetic waves at the desired frequency are taken out.
Conventionally, the dielectric resonator filter of the type that feeds power by means of a microstrip line, which is able to be reduced in size and surface mounted on a circuit board, conversely has a defect that its Q value is small (refer to, for example, Akira Enokihara and three others, “26-GHz band TM11δ-mode dielectric resonator filter”, Mar. 13, 2002, The Radiation Science Society of Japan, Technical Report (RS01-16).
On the other hand, a dielectric filter, which employs a dielectric multilayer structure formed by layering a plurality of dielectrics as the filter element, is known, and it is practical to alternately bond and layer two different types of dielectric ceramic materials with an epoxy adhesive to produce a waveguide type short-circuiter (refer to, for example, Japanese patent publication No. H10-290109 A). The above dielectric filter is an example in which a dielectric multilayer mirror that utilizes multiple reflection used in optics is applied to the millimeter-wave band.